Conventional disk devices are provided with power states, which include an active mode for performing processing in accordance with commands, and a power saving mode for reducing power consumption, without performing any processing, as compared with the active mode. For example, when the continuation time of no processing being performed reaches a certain set value in the active mode, the active mode is shifted to the power saving mode. However, in the case of conventional disk devices, when receiving a command, in the power saving mode, from, for example, an external device, the disk devices return a response after returning into the active mode, and thus take a long time for the response.